The fire barrier properties of building materials and the maintenance of power and communication in fire situations are critical to the safety of inhabitants and effective fire fighting. As a result many countries set standards for the performance of buildings under fire conditions. For example cables for critical applications are required to continue to operate under fire conditions to ensure the maintenance of power and communications. To meet some of the Standards cables must maintain circuit integrity when heated to a specified temperature (e.g. 650, 750, 950 or 1000° C.) for a specified period of time. It is also necessary to take into account that in order to be effective, fire insulation may need to provide protection from the effects of water jet sprays and turbulent gas flows encountered under fire conditions.
It is also desirable that a material used to impart fire resistance has acceptable mechanical strength for the intended application, following exposure to the elevated temperatures likely to be encountered in a fire situation, so that it can remain in place when subjected to the mechanical shocks and/or forces (eg from strong gas currents) associated with fire scenarios.
One method of improving the high temperature performance of an insulated cable has been to wrap the conductor of the cable with tape made with glass fibres and coated with mica. Such tapes are wrapped around the conductor during production and then at least one insulating layer is applied. Upon being exposed to increasing temperatures, the outer layer(s) are degraded and fall away, but the glass fibres hold the mica in place. These tapes have been found to be effective for maintaining circuit integrity in fires, but are quite expensive. Further, the process of wrapping the tape around the conductor is relatively slow compared with other cable production steps. Wrapping tape around the conductor slows overall production of the cable, again adding to the cost. A fire resistant coating that can be applied during the production of the cable (for example by extrusion) thereby avoiding the use of tapes, is desirable.
A variety of materials have been used to impart fire resistance to structures and components, including electric cables. The use of compositions based on silicone elastomers has been reported. However, silicone elastomers can be expensive, have relatively poor mechanical properties and can be difficult to process, for example by extrusion techniques. Furthermore, these compositions tend to have the associated disadvantage that they are converted to powdery substances when exposed to fire as a result of the organic components of the silicone elastomers being pyrolised or combusted. The pyrolysis or combustion products are volatilised and leave an inorganic residue or ash (silicon dioxide) that has little inherent strength. This residue is generally not coherent or self-supporting and indeed is often easily broken, dislodged or collapsed. This behaviour mitigates against using silicone elastomers in passive fire protection. This means, for instance, that silicone polymers used as insulation on electric cables must be protected and held in place with physical supports such as inorganic tapes and braids or metal jackets.
We have found that some materials, based on silicone polymers or other polymers in combination with various inorganic additives, retain their integrity and form self-supporting ceramics on exposure to fire, and some of these have been proposed for use as insulation layers on electric cables that do not contain physical supports.
International Application PCT/AU03/00968 describes a fire resistant composition, which comprises a silicone polymer along with mica, and a glass additive in respective amounts from 5% to 30% and 0.3 to 8% by weight based on the total weight of the composition.
International Application PCT/AU03/01383 (the contents of which are herein incorporated by reference) describes a composition which contains an organic polymer, a silicate mineral filler and a fluxing agent (or precursor resulting in a fluxing agent) to result in from 1 to 15% of fluxing agent by weight of the residue resulting from fire conditions.
Other fire barrier compositions attempt to meet fire rating requirements by using inorganic materials which foam under the influence of a chemical intumescing agent during a fire. For example Horacek (US Pub 2003/0031818 and 2003/0035912) describes an intumescent strip and sheath for wires and cables which forms fire resistant glass foam under fire conditions. The intumescent component is a mixture such as dipentaerythritol, melamine and ammonium polyphosphate in specific proportions which constitutes from 20 to 35% by weight of the total composition. Keogh (US Pub 2002/0098357) describes an intumescent wrap for cables and the like in which the intumescence is provided by a 50:50 blend of ammonium phosphate and melamine. Thewes (US Pub No 2004/0051087) and Rodenberg et al (DE 103 02 198) disclose a fire protection material which likewise rely on the presence of a melamine as a blowing agent to provide foam in fire conditions. The intumescing agents such as melamine and pentaerythritol react with the polyphosphoric acid to form transient phosphate ester species which dehydrate to provide an organic foam.
Commercially available flame retardants may contain a mixture of a phosphoric acid generating agent, a charring agent such as pentaerythritol or carbohydrates and agents such as melamine which accelerate foaming. When mixed in specific proportions the composition provides intumescence. While the formation of foam provides improved insulation and a heat barrier we have found that the expansion generally results in a very mechanically weakened residue which is not self-supporting. As a result of the residue not being self-supporting the insulation is prone to fall or fracture thereby exposing the insulated material. Also the composition is more susceptible to compromising insulation in the presence of water and/or severe air currents which are frequently encountered during fires.